In mixed voltage applications, often, an input/output (I/O) driver may need to be able to drive and receive voltages above a maximum chip operating voltage. For example, a 1.5 volt technology driver may need to drive and receive 3.3 volts. Since 3.3 volts is too high to be applied to 1.5 V I/O FETs directly, semiconductor chips that include the I/Os and the FETs typically utilize additional circuitry, device, and/or process solutions to avoid high-voltage stress across terminals of the FETs, and especially the thin gate oxide of the FETs.
One example of such solutions includes employing two types of FETs that have different gate oxide thicknesses. According to such a solution, one of the FETs is a "normal" FET, such as an NFET or a PFET, that includes thin oxide for low voltage applications. The other FET includes a thicker gate oxide such that the FETs can withstand a much higher voltage across the oxide. FIG. 1 illustrates a schematic of a mixed voltage driver and receiver.
Compared to a mixed voltage driver utilizing normal FETs, the thick oxide driver has a much denser layout. However, thick oxide devices have some disadvantages. For example, they have sub-optimal current drive capability due to high equivalent thick gate oxide and inherent higher threshold voltages. To obtain reasonable performances, the FETs typically need to be made large.
FIGS. 1a and 1b represent schematic diagrams of a known mixed voltage driver and receiver, respectively. These two device structures both suffer from the above-described problems. A thin oxide mixed voltage driver is disclosed in U.S. patent application Ser. No. 08/905,983, filed Aug. 5, 1997, for "Decoupling Scheme For Mixed-Voltage Integrated Circuits", to E. J. Nowak and M. H. Tong, assigned to the assignee of the present application, the entire disclosure of which is hereby incorporated by reference.